The effect of duration of ischemia on impedance in the caudate and cortex have been extensively studied in the cat and correlated with cerebral blood flow, edema, as well as other physiological parameters. It was found that when the cerebral blood flow was reduced to values below about 12 ml/100gm/min the cerebral impedance increased quickly within about 3 minutes to 150% or more of the pre-ischemic value. The magnitude of this rise was variable but the course was characteristic. After about 10 minutes the impedance rise leveled off to a plateau value. Upon release of the occlusion, the impedance rise leveled off to a plateau value. Upon release of the occlusion, the impedance fell within a few seconds towards pre-ischemic values. For short durations of ischemia (less than 10 minutes) the impedance fell to pre-ischemic values. For somewhat longer durations (about 20 minutes) the post-ischemic impedances values were significantly less than the pre-ischemic values. Confirmation that this lowered impedance was indicative of edema was found in lowered specific gravity of the brain tissues. Longer ischemic durations (one hour, and up to three hour durations) showed much variability in the course of post-ischemic impedances. However, all were found to be severely edematous by gravimetric analysis and showing severe leakage of Evans Blue dye through the blood-brain barrier. In some instances, a secondary rise in impedance appeared several hours after release of the occlusion and were believed to reflect a compression of the brain by edematous swelling. The relationship of changes in impedance to ischemia is not yet clear. Further study is planned.